A switched reluctance machine (SRM) is a brushless, synchronous machine having salient rotor and stator poles. There is a concentrated winding on each of the stator poles, but no windings or permanent magnets on the rotor. The SRM can have various numbers of stator poles and rotor poles with the rotor having fewer poles than the stator. Common types are the 6/4 (6 stator poles, 4 rotor poles) and the 12/8. The description to follow, for simplicity, assumes a 6/4 three-phase SRM in which pairs of diametrically opposite stator pole windings are connected in series or in parallel to form an independent machine phase winding of the multi-phase SRM. Ideally, the flux entering the rotor from one stator pole balances the flux leaving the rotor from the diametrically opposite stator pole, so that there is no mutual magnetic coupling among the phases.
Torque is produced by switching current in each phase winding in a predetermined sequence that is synchronized with angular position of the rotor. In this way, a magnetic force of attraction results between the rotor poles and stator poles that are approaching each other. The current is switched off in each phase before the rotor poles nearest the stator poles of that phase rotate past the aligned position; otherwise, the magnetic force of attraction would produce a negative or braking torque. Hence, by properly energizing the phase windings relative to rotor angle, forward or reverse operation and motoring or generating operation can be obtained. Typically, the desired phase current commutation is achieved by feeding back the rotor position signal to a controller from a shaft angle transducer, e.g., an encoder or a resolver. To improve reliability and to reduce size, weight, inertia, and cost in such drives, it is desirable to eliminate this shaft position sensor. To this end, various approaches have been previously proposed for indirect rotor position sensing by monitoring terminal voltages and currents of the machine. One such approach, referred to as waveform detection, depends upon back electromotive forces (emf) and is, therefore, unreliable at low speeds and inoperative at zero speed.
Another approach to indirect rotor position sensing is disclosed in commonly assigned U.S. Pat. No. 4,772,839, issued Sep. 20, 1988 to S .R. MacMinn and P. B. Roemer, which patent is incorporated by reference herein. The cited patent describes an indirect position estimator for an SRM which applies low-level sensing pulses of short duration to the unenergized machine phases. Application of the sensing pulses results in a change in current in each of the unenergized phases. The change in current is sensed by a current sensor and an estimated inductance value is derived therefrom. A pair of estimated rotor angles corresponding to the estimated inductance value for each of the unenergized phases is ascertained. One such pair is shifted by a value equal to a known phase displacement of the other unenergized phase. The pairs of estimated angles are then compared to determine which of the angles match. An estimated instantaneous rotor angular position equal to the matching angle is produced. Moreover, in case any of the stator phase undergoes a change in state during sampling or in case two phases do not remain energized throughout the sampling, an extrapolator is provided to generate an extrapolated rotor angular position instead of the estimated position.
Still another approach to indirect rotor position sensing is disclosed in commonly assigned U.S. Pat. No. 4,959,596, issued to S. R. MacMinn, C. M. Stephens and P. M. Szczesny on Sep. 25, 1990, which patent is incorporated by reference herein. According to U.S. Pat. No. 4,959,596, a method of indirect rotor position sensing involves applying voltage sensing pulses to one unenergized phase. The result is a change in phase current which is proportional to the instantaneous value of the phase inductance. Proper commutation time is determined by comparing the change in phase current to a threshold current, thereby synchronizing phase excitation to rotor position. Phase excitation can be advanced or retarded by decreasing or increasing the threshold, respectively.
A still further approach to indirect rotor position sensing is disclosed in commonly assigned U.S. Pat. No. 5,107,195 issued to J. P. Lyons, S. R. MacMinn and M. A. Preston on Apr. 21, 1992, the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 5,107,195 discloses a method and apparatus for indirectly determining rotor position in a switched reluctance motor (SRM) based on a flux/current model of the machine, which model includes multi-phase saturation, leakage, and mutual coupling effects. The flux/current model includes a network mesh of stator, rotor and air gap reluctance terms. The network is driven by magnetomotive force (mmf) terms corresponding to the ampere-turns applied to each of the stator poles. Phase current and flux sensing for each phase are performed simultaneously. The reluctance terms of the flux/current model are determined from the phase flux and current measurements. The phase current and flux measurements also determine the rotor position angle relative to alignment for each respective motor phase and which phase (or phases) is operating in its predetermined optimal sensing region defined over a range of rotor angles. The measurements on at least two phases are then used for establishing whether the stator phases of the sensing phase are approaching alignment or maximum unalignment with SRM rotor poles. Finally, the rotor position angle for the sensing phase and its position relative to alignment are used to provide a rotor position estimate for the motor.
While the method of U.S. Pat. No. 5,107,195 is preferred for determining rotor angle or position, that method is not believed sufficiently accurate at start-up or low speed ( less than about 100 RPM) operation of a 25,000 RPM SRM. Whether one uses either a flux/current map, e.g., look-up table, or solution of a lumped parameter flux/current model, the position estimate is derived from stator flux-linkage and current of the torque-producing phases with stator flux being estimated by integrating the quantity (V-IR), where V is the applied phase voltage, I is the phase current and R is the winding resistance. However, the technique is ineffective at low rotational speeds since the torque-producing current pulses are of sufficient duration to allow for significant error to accumulate in the flux integrators thus rendering the derived position estimates meaningless.
All of the methods described for determining rotor angle or position in an SRM rely on characteristics of the switched reluctance machine. For example, saliency in both the rotor and stator of a switched reluctance motor causes the SRM to have an air gap of varying effective area, and thus the phase inductance seen from the terminals of the stator phase windings is a strong function of rotor position. The current in one phase winding of a switched reluctance motor and the flux linked by that winding are related by the winding inductance through the relationship .psi.=Li. The flux current methods for SRM rotor position estimation exploit the inherent magnetic characteristics of the SRM flux path to infer the rotor angular position.
For each SRM phase the stator flux-linkage is estimated by .psi.=.intg.(V-IR), where V is the applied phase voltage, I the phase current, and R is the winding resistance. Then, given estimated .psi. and measured I, the rotor position relative to alignment for each of the SRM phases can be obtained from the magnetic characteristic, or flux-current map for various rotor angles. This non-intrusive method monitors the normal torque-producing voltage and current waveforms in order to infer the rotor position. Additional logic then chooses the best available relative angle measurement and subsequently translates the relative rotor angle obtained from the magnetic characteristic into the absolute rotor angle required for commutation control of the SRM. U.S. Pat. No. 5,097,190 describes this flux-current map technique.
The flux-current map technique for determination of SRM rotor angle utilizes a single-phase magnetic characteristic as an underlying model. This model assumes that only the sensing phase is conducting current or that mutual coupling effects between conducting phases are negligible. For many applications neither of these assumptions are valid. A lumped parameter flux/current model for a three-phase switched reluctance machine can be utilized to account for mutual coupling between phases. In order to predict the rotor angle using this multi-phase model, it is necessary to sample all phase currents and flux-linkage estimates simultaneously and then solve the reluctance mesh equations to isolate the gap-tip reluctance terms R.sub.gt (.theta.,.phi.). The gap-tip reluctance function, at a known rotor flux level, can then be inverted to yield the relative angle to alignment .theta. for each of the stator poles--the inverse gap-tip reluctance function will most commonly be stored as a two-dimensional characteristic. The optimal absolute rotor position estimate is again obtained via post-processing logic. U.S. Pat. No. 5,107,195 describes this lumped parameter technique.